To protect a vehicle occupant from the impact of a vehicle crash, air bag systems are increasingly widely installed in motor vehicles. Typically, such an air bag system is incorporated with an inflator for producing the gas required for inflating the air bag. The inflator is compactly folded and stowed in the center of the steering wheel or in the dashboard which is located immediately in front of the vehicle occupant. The inflator contains a propellant which, upon ignition, rapidly burns and produces the gas required for rapidly deploying the air bag. An inflator of this type is disclosed, for instance, in Japanese patent laid-open publication (kokai) No. 5-319199.
In such an inflator, because a substantial amount of the thermal energy produced from the initial combustion of the propellant is dissipated to the housing of the inflator and the air bag, some initial time delay in deploying the air bag is inevitable. This is particularly pronounced because of the fact that the combustion of the propellant progresses at a constant rate, and the initial build up of the pressure is not thermodynamically efficiently utilized. Such a time delay may be compensated for by increasing the amount of the propellant used, but it leads to an undesirable increase in the size and weight of the inflator.
To overcome such a problem, it is conceivable to control the rate of the combustion of the propellant. For instance, the housing may be divided into two chambers so that the propellant of the first chamber, which is normally smaller than the second chamber, is ignited first, and that the resulting flame is introduced into the second chamber, via communication holes provided in the side wall separating the two chambers, to ignite the propellant in the second chamber. The combustion gas resulting from the combustion of the propellant in the first chamber is used for preheating the housing and the air bag, and for slightly inflating the air bag. The combustion of the propellant in the second chamber produces the gas at such a rate that is required to rapidly deploy the air bag.
The housing for an inflator assembly serves as a pressure vessel, and the interior of the housing is required to be divided into a number of separate chambers. Also, the ignition pipe for producing the ignition flashes is required to be accurately positioned inside the housing. Therefore, the structure of the housing tends to be complicated, and this undesirably increases the manufacturing cost.
Conventionally, the housing was provided with a circular cross section so as be able to withstand the pressure which develops inside the housing when the propellant burns. However, the inventors have discovered the fact that a circular cross section is detrimental to the optimum performance of the inflator. Because the produced gas is required to be directed in a specific direction, the perforation which may be formed on one side of the housing doe not provide a sufficient passage area. Also, because the outer wall of the inflator housing is generally curved, it is difficult to form a large number of holes for gas ejection in an economical manner with a required precision.
The outlet of the gas from the inflator assembly is required to have a filter to prevent solid residues and other foreign matters from being projected toward the air bag. It is therefore essential to employ a favorable filter structure for the manufacturing cost to be minimized while the performance of the inflator assembly is maximized.